1. Field of the Invention
This invention relates generally to gas analyzers and more specifically to apparatus and methods for calibrating gas analyzers such as those commonly used for analyzing automotive exhaust.
2. Description of the Prior Art
Gas analyzers of the prior art are used for two general purposes. In some analyzers, it is desired to determine the unknown components of a particular gas mixture. In other gas analyzers, the components are known and it is desired to determine the percentage of concentration of the known gas component within the gas mixture. It is this latter type of analyzer which is commonly used for determining the amount of carbon monoxide, hydrocarbons, carbon dioxide, etc. present in the exhaust gas of an automobile.
It is generally well known that gases have properties for absorbing infrared energy at different wavelengths. Thus, in the first type of analyzer, infrared energy is introduced into the gas mixture and the energy emanating from the gas mixture is measured at different wavelengths. If the output energy is particularly low at a given wavelength, this is indicative of the presence in the gas mixture of the particular gas associated with that given wavelength.
In the second type of analyzer, the concentration of a particular gas in the gas mixture is of interest. In such an analyzer filters are typically used to pass only the energy present at the absorption wavelength associated with the particular gas. In this manner, the energy measurements can be limited to wavelengths narrow enough to exclude other gases which might interfere with the measurement of energy absorbed by the particular gas of interest.
One of the first problems associated with this method of analysis results from the fact that a change in the amount of energy leaving the gas cell can be caused either by the presence of the particular gas within the mixture, or by a change in the amount of energy introduced into the mixture. There has been no simple way to tell which circumstance caused the change in the output energy. This is further complicated by the fact that gas analyzers are typically calibrated as that relatively small changes in output energy correspond to full scale gas concentrations. For this reason, small percentage changes in input energy to a cell could be confused with large concentrations of the particular gas in the mixture. In addition, the amount of infrared energy introduced into the sample varies widely as the temperature of the infrared source changes.
To aid in the elimination of this problem, double beam instruments have been constructed to include one channel containing a gas cell into which the gas mixture is introduced. A reference cell is included in another channel but only air or some inert gas is present in the reference cell. Thus, the energy at the output of the gas cell is related to the amount of energy introduced into the gas cell and is also related to the concentration of the particular gas in the mixture. The energy present at the output of the reference cell is generally related only to the amount of energy entering the reference cell. Thus, the energy signals at the outputs of the cells differ generally only due to the presence of the particular gas in the gas cell.
In these analyzers, the energy signals have been introduced to a detector which provides a composite electrical signal. These composite signal has been processed to produce a reference signal having a magnitude proportional to the energy emanating from the reference cell, and to produce a gas signal having a magnitude proportional to the energy emanating from the gas cell. The gas and reference signals have been introduced to a differential amplifier to produce a difference signal having a magnitude indicative of the loss of energy resulting from the presence of the gas mixture in the gas cell. This loss of energy is dependent upon the volume of the particular gas in the mixture and a meter is typically scaled to provide that indication.
One of the problems associated with the analysis of gas samples results from the fact that the actual percentage of energy absorbed, unfortunately, is not related to the concentration of the gas in the cell, but rather to the number of molecules of gas in the cell. As gas is comprised of free moving molecules which fill a space so that the number of molecules varies significantly with the temperature and pressure of the gas.
The magnitude of the electrical signal also tends to vary with several factors other than the percentage of concentration of the particular gas. For example, the emissivity of the energy source typically varies over a period of time so that the amount of energy entering the cell tends to decrease with age. Also, the sensitivity of the detector varies dramatically with the ambient temperature. Furthermore, the amount of energy absorbed along the optical path tends to increase if dust is permitted to build up in the analyzer. For these reasons, it is desirable to calibrate gas analyzers prior to each use in order to compensate for these factors.
One method for calibrating these instruments has been to initially analyze a gas of known concentration, such as a gas including ten percent carbon monoxide. Analyzers using this method are typically provided with two calibration knobs on the face of the instrument. The first knob is used to zero the meter prior to the introduction of the calibration gas into the instrument. The second knob is used to adjust the span of the meter to indicate the known concentration of the particular calibration gas. Thus, two steps have been used to calibrate the instruments of the prior art.
This calibration method is particularly accurate since it compensates for substantially all of the factors mentioned above. However, this method has been relatively inconvenient. For example, it has been difficult to make the gas available wherever the analyzers have been used. In a laboratory, the calibration gas can be easily stored in proximity to the instrument, but it has been particularly inconvenient to transport this calibration gas for use in the field. Furthermore, the calibration gas has been relatively expensive.
A simpler, although less accurate, method of calibration has involved the use of an obstruction, commonly referred to as an opacity, for use in blocking a known portion of the infrared energy prior to its introduction into the gas cell. This, of course, reduces the magnitude of the energy emanating from the cell. In fact, the amount of the energy reduction is related to the size of the opacity. For calibration purposes, the opacity has been sized to provide the same reduction of energy that a known quantity of gas molecules would provide at a given temperature and pressure. However, the opacity at a constant size corresponds to a different concentration at different altitudes and different temperatures so that this method of calibration does not solve the gas density problem. Nonetheless, if the analyzer is used in a single location and a temperature controlled environment the gas density problem is not of particular significance.
Unfortunately, the placement of the opacity within the infrared energy stream can have different effects if the energy gradient varies across the stream. In some cases, the opacity has been provided with a configuration of a comb to average the energy gradient, but dust collecting on the teeth of the comb has tended to degrade the accuracy of the calibration. Means have been provided to precisely locate the opacity in a specific position within the energy stream. However, these means have been relatively expensive.
As a result, the calibration methods of the prior art have been inconvenient, inaccurate, expensive and time consuming. Furthermore, small deviations in the accuracy of the calibration have produced significant changes in the gas concentrations indicated by the meter.